A challenging aspect of treating those who have lost one or more limbs is the restoration of function to the remaining limb (residual limb). Such a restoration often means fitting the residual limb with one or more prostheses, or artificial limbs. A difficulty with artificial limbs is that they often do not physically interface well with the amputee. For instance, most prostheses comprise a cup into which the residual limb, protected with an elastomeric sheath or “liner,” is inserted and secured. Since the 1980's, prosthetic liners have generally been of polymeric materials. Such liners are described in U.S. Pat. Nos. 5,549,709; 6,645,253; 6,761,742; 6,554,868; 6,726,726; 6,926,742 and 6,974,484.
Current liner designs available to amputees are generally simple. Many are based upon a single-layer elastomeric sleeve, usually with a slight taper from the open end to a blunted closed end. Because of such simplicity of design, the liners can be manufactured in large numbers from easily worked materials at a relatively low cost and without the need for extra steps or machinery.
However, amputees who have been fortunate enough to retain limb joints, such as knee or elbow joints, can find the relatively simple design of commonly available elastomeric liners to be inadequate. For example, amputees having below-the-knee prosthetics generally require a liner having the ability to conform to a range of conformations accessed by an intact, functioning knee joint. A problem with many commercially available liners is that their simple construction and design, largely dictated by ease and relatively low costs of manufacture, generally fails to conformationally accommodate the knee joint as it undergoes a full range of motion. Simple sleeve-type liners often do not provide a comfortable fit over the entire range of motion of the joint. Even at small bending angles, the fit of the liner behind the knee can be lost due to bunching or gathering behind the knee (“buckling”). Buckling is the development, upon flexion of the knee joint, of one or more folds in the portion of the liner overlying the region behind the knee (popliteal fossa). The folds generally occur in a lateral direction, i.e., roughly perpendicular to the length of the leg. However, more complex, crinkle-type folds can also occur. The pinching and pulling of underlying skin which can occur with buckling can result in patient discomfort. Moreover, many liner-wearers also experience sweating behind the knee, which can exacerbate the discomfort caused by buckling.
As the knee joint undergoes flexion, the relatively relaxed liner surface disposed over the kneecap (the “anterior surface”) must stretch and bend in order to accommodate the change in conformation of the knee joint, as well as the increase in anterior skin surface area which accompanies the change. The maximum stretch occurs when the knee joint is at its fully bent position. The liner surface behind the knee (the “posterior surface”) must also accommodate changes in the conformation of the joint during bending. However, unlike the anterior surface, it must accommodate a reduction in the vertical dimension as it takes on a bent conformation. Upon joint flexion, while the anterior surface can easily stretch and still smoothly overlay the skin, the posterior surface is essentially required to assume a “contracted” conformation in order to continue to smoothly overlay the area behind to the knee.
When elastomeric liners flex with the knee, the liner portion disposed over the knee cap (the “anterior portion”) stretches in the longitudinal direction (i.e., along the length of the leg, and contracts to a degree in the transverse direction (i.e., across the front of the leg, and in a direction generally perpendicular to the longitudinal direction). In order to maintain a buckle-free fit, the liner portion overlying the area behind the knee (the “posterior portion”) preferably will stretch in the transverse direction and contract in the longitudinal direction. The transverse contraction of the anterior portion will aid in stretching the posterior region transversely to a degree, which, in turn, will cause the anterior region to contract longitudinally to a degree.
It should be noted that substantially thickening the posterior region can have the affect of simultaneously reducing buckling and impeding the aforementioned contraction. However, such a solution is generally not conducive to wearer comfort in that it can increase the stiffness of the liner and impede its ability to undergo the angular bend required by flexion.
Current polymeric/elastomeric gel liner materials having the suppleness necessary for wearer comfort generally easily accommodate the anterior stretching, but posteriorly, such materials can be prone to buckle upon flexion. Efforts to eliminate buckling by fabricating liners from materials having increased “stiffness,” such as by using materials having low elongation, tend to reduce the ability of the liner overlying the anterior portion of the knee to stretch and accommodate flexion of the knee joint, leading to increased wearer discomfort. Furthermore, the fabrication of a liner having anterior characteristics different from posterior characteristics, such as, for example, a continuous but non-homogeneous liner, or a liner fabricated by edge-joining two or more different elastomeric surfaces is generally a poor solution in that it can be costly, and require a seam between the elastomeric regions with differing characteristics. Such a seam in a monolayer liner can be a zone of weakness and undesirable from the standpoint of liner integrity.
The use of material overlays in the posterior region to “stiffen” the liner by appreciably increasing the overall thickness of the liner overlying the posterior region can interfere with the ability of the posterior region to responsively conform during flexion of the knee joint. Wearer discomfort is generally increased.
Thus, there is a need to produce comfortable liners which easily accommodate the wide range of motion of the knee joint without unnecessary posterior or anterior stiffness, and exhibit reduced or no buckling behind the knee upon flexion.